Knock back (6)
(Other definitions for rebuff that I've seen before include "Snub or contemptuous refusal" , "Deliberate discourteous act" , "Reject abruptly" , "Refuse" , "Abrupt or ungracious rejection" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">knock back</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that knock back can be written as refuse)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for refuse that I've seen before include "deadwood" , "Turn away" , "Junk" , "Say no to - waste" , "Decline to accept garbage" .)